1. Field of the Invention
The present invention relates to a glass for a near infrared absorption filter, a near infrared absorption filter formed of the glass, and a solid-state image sensing device to which the filter is applied. More specifically, it relates to a glass for a near infrared absorption filter, which has spectral characteristics that it efficiently transmits light in a wavelength region of from ultraviolet region to 600 nm and selectively absorbs light in a near infrared region of a wavelength of larger than about 600 nm and which is suitable particularly for use for the color correction of a color VTR camera, a near infrared absorption filter which uses the above glass and is suitable for the above use, a solid-state image sensing device to which the filter is applied.
2. Explanation of Related Technology
As a solid-state image sensing device, conventionally, a color VTR camera mainly uses CCD (charge coupled device) or MOS (metal oxide semi-conductor). Generally, the solid-state image sensing device has a spectral sensitivity to light having a wavelength of approximately from ultraviolet region to 1,000 nm, and the sensitivity extends to a near infrared region, so that it is required to correct the above sensitivity so as to meet the luminous efficacy of human eyes. When natural light comes into the image sensing device, a formed image comes to be reddish and no good color reproduction is attained. In general practice, therefore, light which comes into the image sensing device is corrected with a filter which absorbs light in the near infrared region.
As a glass for use as the above near infrared absorption filter, a variety of glasses obtained by incorporating CuO as a coloring component into a phosphate glass have been developed and are practically used.
For example, there have been disclosed (1) a colored glass containing, by % by weight, 60 to 90% of P.sub.2 O.sub.5, 7.5 to 20% of Al.sub.2 O.sub.3, 0 to 15% of a total of B.sub.2 O.sub.3 and SiO.sub.2, 1 to 25% of a total of BaO, MgO, CaO and SrO, 0 to 15% of a total of Y.sub.2 O.sub.3, La.sub.2 O.sub.3, ZrO.sub.2, Ta.sub.2 O.sub.5 and TiO.sub.2, 0 to 10% of PbO and 0.4 to 15% CuO (JP-A-62-128943), (2) a phosphate glass containing, by % by weight, 60 to 80% of P.sub.2 O.sub.5, 3 to 11% of Al.sub.2 O.sub.3, 3 to 9.5% of BaO, 3 to 20% of a total of MgO, CaO, BaO and SrO, 1 to 5.5% of Li.sub.2 O, 1 to 10% of a total of LiO.sub.2, Na.sub.2 O and K.sub.2 O, 0 to 5% of a total of SiO.sub.2 and B.sub.2 O.sub.3, 0 to 5% of ZrO.sub.2, TiO.sub.2, Y.sub.2 O.sub.3 and La.sub.2 O.sub.3 and 0.2 to 10% of CuO (JP-A-4-104918) and (3) a phosphate glass containing, by % by weight, 35 to 50% of P.sub.2 O.sub.5, 0 to 5% of Li.sub.2 O, 0 to 12% of Na.sub.2 O, 0 to 20% of K.sub.2 O, 0 to 20% of Cs.sub.2 O, 1.5 to 20% of R.sub.2 O (in which R is an alkali metal), 0 to 48% of ZnO, 0 to 7% of MgO, 0 to 7% of CaO, 0 to 7% of SrO, 0 to 12% of BaO, 0 to 15% of R'O (in which R' is an alkaline earth metal) and 0.2 to 12% of CuO (JP-A-9-100136).
Since, however, the above glasses (1) and (2) contain a relatively large amount of P.sub.2 O.sub.5, a glass component may be eluted when they are used for a long period of time, and it is therefore difficult to use them for a long period of time. Further, when a relatively large amount of Al.sub.2 O.sub.3 is incorporated for improving climate resistance, the melting temperature thereof increases, and as a result, Cu.sup.2+ is reduced and Cu.sup.+ having an absorption in an ultraviolet region decreases the transmittance to light at and around 400 nm. There is therefore a defect that such a glass is undesirable as a sensitivity correction filter for CCD.
In the above glass (3), the content of P.sub.2 O.sub.5 is decreased so as to be relatively smaller than that in a conventional phosphate glass, a relatively larger amount of ZnO is incorporated, and an alkaline earth metal oxide and an alkali metal oxide are added as required, whereby the climate resistance and the high transmittance to light in ultraviolet to visible light regions, which cannot be obtained with a conventional phosphate glass, can be obtained.
However, the above glass is not necessarily fully satisfactory although it is relatively fine concerning its alkali resistance.
When a glass has excellent alkali resistance, it is advantageous in that it is free from latent damage even when it is washed with an alkaline detergent highly capable of washing after the glass is polished.